The present invention relates generally to the field of industrial devices which are useful for heating particulate material and, more particularly, to such devices which are useful in the production of litharge.
Litharge (lead monoxide) is supplied in industrial quantities for use in the pigment industry and in the leaded glass industry. Litharge is produced by oxidation of particulate lead into lead monoxide. Presently, there are two methods employed in the production of litharge. The first method uses a rake furnace to oxidize lead in a batch process. The lead or "leady oxide" (PbO+Pb) is heated and stirred by means of a rotating rake. The second method is a continuous production process which employs a horizontal drying kiln in which the lead is tumbled while being oxidized.
Both the methods described above suffer certain disadvantages in use in that the particulate material does not get evenly heated, resulting in the undesired presence of lead and "red lead" in the final product. Moreover, in certain applications, such as the leaded glass industry, it is important that the litharge meet certain particle size distribution specifications. Since the input raw material does not meet such specifications, the final litharge product must be milled to meet the desired particle size specifications. While the mixing and blending action which takes place using the above described methods does serve to somewhat reduce the particle size distribution, considerable milling operations are still required.
Each of the following references disclose various types of rotary furnaces for heating particulate material: U.S. Pat. No. 3,763,013 to Allred; U.S. Pat. No. 4,412,813 to Wulf; U.S. Pat. No. 4,449,924 to Ceretti; U.S. Pat. No. 1,208,248 to Wedge; and U.S. Pat. No. 1,064,516 to Miller. None of these references, however, disclose the use of rotating helical conveyors in a rotary furnace.